Experimental reflectance study of methane and ethane ice at Titan’s surface conditions

Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) has provided evidence of several different hydrocarbons on the surface of Titan using seven atmospheric windows. Methane (CH 4 ) and ethane (C 2 H 6 ) are suggested to exist in both the liquid and solid states on Titan. Even if the average surface temperature (90–94 K) of Titan is clearly in the liquid stability field of both CH 4 and C 2 H 6 , the particles can condense in the atmosphere (∼65 km for C 2 H 6 and lower stratosphere for CH 4 Anderson et al. in Icarus, 243:129–138, 2014 ) and precipitate allowing them to melt and/or sublimate. It is also suggested that these liquids can freeze on the surface due to evaporative cooling. We conducted a laboratory study at Titan surface conditions to determine the phase change of CH 4 and C 2 H 6 and to test if they would freeze on the surface of Titan. Using NIR reflectance spectroscopy, we calculated the reflectivity ratio ( R solid / R liquid ) of CH 4 and C 2 H 6 of 1.08 and 1.36, respectively, suggesting an 8% increase in reflectivity for CH 4 and a 36% increase for C 2 H 6 during phase change. The low albedo in liquid phase for both CH 4 and C 2 H 6 is consistent with observations made by VIMS in both Titan’s northern and southern latitudes. We also find the evaporation rate of amorphous CH 4 close to Titan conditions, which is 9.0 ± 0.3 × 10 − 5 kg s − 1 m − 2 at 87 K and we estimated a sublimation rate of 0.22 × 10 − 5 kg s − 1 m − 2 at 83 K for a 1.5 bar N 2 atmosphere. The freezing rate of ∼46 m/year for C 2 H 6 was observed whereas, for CH 4 , we observed that CH 4 does not freeze at 87 K due to a high N 2 dissolution rate. However, the viscosity of CH 4 increases with a decreasing temperature that results in amorphous CH 4 . The results show a remarkable difference between the formation of ice for two liquids with different N 2 dissolution rates. Consequently, using the results obtained from the laboratory study we predict that the observed change is albedo during and after rainfall on Titan is caused by CH 4 due to evaporative cooling processes.